1. Field of the Invention
The present invention relates to an improvement in a semiconductor laser of a terraced substrate type.
2. Description of the Prior Art
A semiconductor laser has the advantage of smallness of bulk, high efficiency and direct modulation by means of its current, and therefore has a bright future as light sources for optical communication, optical data processing. A laser for such use necessitates characteristics of stable fundamental transverse mode lasing, low threshold current, high output of light and high reliability.
The conventional laser which has a structure of simple gain guiding has a difficulty in maintaining a transverse mode for a wide range of current, and therefore is liable to occurrence of undesirable mode conversion or a generation of higher modes. As a result of these, the light-current characteristic is likely to have a kink of characteristic curve or the device is likely to have a multiple lengthwise mode oscillation.
FIG. 1, FIG. 2 and FIG. 3 show various types of a conventional semiconductor stripe laser, wherein FIG. 1 shows a planar stripe laser. The laser of FIG. 1 has a doublehetero structure which has on
a substrate 1 of . . . n-GaAs PA0 a first clad layer 2 of . . . n-GaAlAs, PA0 an active layer 3 of . . . non-doped GaAs, PA0 a second clay layer 4 of . . . p-GaAlAs and PA0 an isolation layer 5 of . . . n-GaAlAs, which forms a p-n isolation junction between it and the underlying p-type second clad layer 4 and has a PA0 stripe shaped current injection region 6 of . . . p-type diffused region formed by diffusing an acceptor such as Zn, in a manner to penetrate it and diffuse into the midway of the second clay layer. PA0 a terrace shaped substrate 11 of . . . n-GaAs, and thereon epitaxial layers of PA0 a first clad layer 12 of . . . n-Ga.sub.1-x Al.sub.x As, an active layer 13 having an oblique PA0 lasing region 131 of . . . (non-doped) Ga.sub.1-y Al.sub.y As, PA0 a second clad layer 14 of . . . p-Ga.sub.1-x Al.sub.x As and PA0 a cap layer 15 of . . . p-GaAs.
Numeral 7 and 8 designate p-side and n-side electrode ohmicly contacting the current injection region 6 and the substrate 1, respectively.
In such planar stripe laser, the active layer has a flat structure and has uniform refractive index on all parts thereof. Therefore, the light confinement in the stripe shaped region is not sufficient. Besides, current injected from the current injection region 6 is likely to spread as shown by the curve I of FIG. 1 and spread parts around both sides of the curve which does not contribute to the oscillation. As the width W becomes narrower until the width W becomes 7 .mu.m, the threshold current gradually decreases. However, it is found that when the width becomes smaller than 7 .mu.m, the threshold current of the laser increases. This is supposed that for the width W of smaller than 7 .mu.m, spreading current which flows at both side parts of the curve I increases in relation to the effectual current which flows at the center of the curve I. Furthermore, when the injected current is increased, the light distribution becomes, as shown by curve II of FIG. 1, that is strong light intensities appear on both side parts, resulting in oscillations on these side parts thereby losing uniformity of oscillation.
FIG. 2 shows a structure of another conventional laser, wherein the substrate 1 has a groove 10 of a stripe shaped pattern and on such substrate 1 a first clad layer 2 and an active layer 3 and known subsequent layers 4 and 5 are formed. In this laser, the stability of single mode oscillation is improved over that the laser of FIG. 1; but the structure of the active layer per se is still flat like the structure of FIG. 1, that is, there is no means to limit spreading of the injected current. Accordingly, its threshold current is not sufficiently reduced.
FIG. 3 shows still another example of the conventional laser, which has been proposed by some members of the inventors of the present invention. One example of this laser has
Thereon are formed insulation layers 16 and 16 of, for example, an SiO.sub.2 having a stripe shaped opening at the position above the oblique active region 131. And electrodes 18 and 17 are formed on the p-side and n-side of the substrate. In this prior art, the active layer has an oblique lasing region 131 defined by an upper bending part and a lower bending part which confines light therebetween, and the first clad layer 12 has a triangular thick part 121 under the oblique lasing region 131 and upper thinner part and a lower thinner part under an upper horizontal part and a lower horizontal part of the active layer 13, respectively. Therefore the thicker part 121 of the first clad layer 12 serves to prevent absorption of light into the substrate 11, while the thinner parts of the first clad layer 12 serves to allow absorption of light therethrough into the substrate 11. Therefore, by the difference of the light absorption from the active layer 13 to the substrate 11, the light is effectively confined in the lasing region 131 which is on the thicker part 121 and between the two bending parts, and thereby a single transverse mode oscillation is easily obtainable, the manufacture thereof is easy and life time thereof is long because of reasonable crystal structure. However, the terraced substrate laser structure of FIG. 3 still has a problem that some part of the injected current still flows into the upper and lower horizontal parts of the active layer 13 resulting in ineffective currents there and confinement of the injected current in the oblique lasing region is difficult, and therefore the external differential quantum efficiency is not sufficiently high. Besides, in order to prevent undesirable parasitic oscillation in the active layer 13 at the other parts than the oblique lasing region 131, the first clad layer 12 should be extremely thin at the parts under the abovementioned "other parts", but to form the clad layer extremely thin is not easy.